A method of controlling compressors operated in parallel in a refrigeration system is known from U.S. Pat. No. 3,527,059. The power for the compressors is regulated in accordance with the empirically determined current of each coolant through its compressor to maintain both currents equal. The result is, assuming equivalent compressors, a uniform load and output for both. This method is not appropriate for dissimilar compressors, and even a uniform load on them will not necessarily result in economical operation.
A similar method is known from French 2 108 039. It is employed to control electrically powered parallel compressors in a refrigeration system. The objective of that method is also uniformity of the load on and output from the individual compressors. The amounts of electricity consumed by each compressor motor are determined and compared, and signals are derived therefrom to control the motors and ensure that each consumes the same amount of electricity. The aforesaid drawbacks occur in this case as well.
A method of operating two compressors in series is known from U.S. Pat. No. 4,255,089. This approach involves distributing the load between the two compressors by means of prescribed data stored in a memory in accordance with a control signal that represents the demands of a downstream system or process. The requisite data are in the form of series of sequences of linear functions. The drawback of this method is that it requires very large memories and, because of the relatively frequent recourse to the memory, is relatively slow and hence inappropriate for more than two compressors. It is also impossible for this method to respond to changes that occur in the compressors as they age, become contaminated, or undergo servicing for example once the memory data have been established. To address these problems would require the very complicated generation and entry of new memory data.
"Control of Parallel Compressors" by A. E. Nisenfeld et al., ISAAC Advances in Instrumentation, 31, 1 (1976), 581.1-585.7 discusses the problems involved in operating two compressors in parallel. Possible approaches to optimizing the operation that are mentioned in this article include the aforesaid uniform load distribution and maximizing the overall efficiency. Dynamic simulation of parallel compressor operation in a hybrid computer is suggested as one way of attaining the latter approach, although no more precise recommendations or concrete technical theories are provided.
Finally, a method of operating at least two turbocompressors is known from European Patent 0 132 487 B1. The core of this method is to match the compressors with load distributors such that the operating points of all the compressors will always be the same distance away from their blowoff line. Only one of the compressors is controlled by pressure regulators, and the others follow. The drawback to this method is that it can assure an approximately optimal operation only for similar compressors and not for different types.